The present invention relates to rotary valves controlled by actuators. Particularly, the invention relates to actuators designed for control valves.
Quarter-turn valves generally applied in the industry are often operated by means of pneumatic actuators. The actuators can be either single-acting or double-acting, whereby with the single-acting devices, movement in the opposite direction is effected by a spring, compressed air working against the spring.
In a double-acting pneumatic actuator, both directions of operation are actuated by compressed air. In that case, sealing must be arranged for two pneumatic pistons, or pistons and piston rods, which causes considerable friction. In shut-off operation the friction causes no great disadvantage, but when using the valve for control, the difference between the static friction and the dynamic friction of the elements of the device must be as small as possible, and the friction must be as low as possible.
High initial-stage friction during movement causes an output surge, in this case a shaft overreaction, when gas is used as pressurized medium. If the difference between the static friction and the dynamic friction is large, movement does not start before the moment caused by the pressure difference exceeds the static friction force and the opposing moment caused by the load. When the elements start moving, the movement continues until the opposing forces due to the pressure difference and to the dynamic friction (which is lower than the static friction) plus the load, are equal.
In a surge situation, small position changes easily cause position overshoot and a corrective motion. Depending on the control gain, oscillation around the required position may also arise in this situation.
In patent publication U.S. Pat. No. 4,610,411, a double-acting actuator for changing reciprocal linear motion into rotational motion is disclosed, said actuator comprising a gear rack in contact with a gear wheel, the gear contact line being concurrent with the line running through the piston centers. The device is an actuator designed for the directing wheels of a landing gear of an airplane, using a piston system sealed with elastic rings and providing, with a special arrangement, a longer travel when required. The construction is rigid and involves considerable friction.
gear wheel, the gear contact line being concurrent with the line running through the piston centers. The device is an actuator designed for the directing wheels of a landing gear of an aeroplane, using a piston system sealed with elastic rings and providing, with a special arrangement, a longer travel when required. The construction is rigid and involves considerable friction.
Attempts have been made to solve friction problems caused by piston-cylinder arrangements by means of diaphragm designs. In U.S. Pat. No. 5,477,642, a double-acting diaphragm actuator designed for automobiles is disclosed, wherein the diameters of pistons are smaller than those of the cylinders, and to the pistons and the walls of the cylinders are fitted diaphragms that provide sealing and flex for the length of the piston travel. Between the pistons there is a gear rack actuating a geared shaft end located in the space between the cylinders. When the actuator performs the transversal motion, this motion can be transmitted to the shaft as such, or converted to rotational movement.
In Russian patent 2097608, a similar double-acting diaphragm actuator is disclosed. Common for these is the feature that the point of contact of the geared shaft end and the gear rack connecting the pistons is located to the side of the line connecting the axis of the pistons. Thus, the push force exerted on the gear rack from the direction of either of the pistons also effects a side thrust tending to move the pistons away from their symmetrical position on the path of movement. Additionally, the non-toothed back of the rack, directed away from the shaft end, is supported against the housing of the actuator, sliding against it, which causes considerable friction.
In U.S. Pat. No. 4,794,847, a single-acting diaphragm actuator is disclosed, in which a gear rack connected to a piston acts on a geared shaft end. A support wheel for the gear rack is provided on the smooth side of the rack. This wheel is required particularly because the spring of the single-diaphragm actuator affects the gear rack asymmetrically. Because the centering of the gear rack must be accomplished with a support wheel, also this design produces friction and the device is not suitable for accurate control applications.
U.S. Pat. No. 5,560,282, DE patent application 2920236 and EP patent application 47 613 also describe actuators provided with sliding pistons.
A diaphragm actuator as described in claim 1 has now been invented, said actuator being characterized in that the gear rack is arranged so that the points of contact between the gear rack and the gear wheel are located substantially on the straight line connecting the centers of the movable walls of the pressure spaces that effect the motion. Therefore, no torsion forces act on the backing plates of the diaphragms or the diaphragms themselves, but the forces effecting the motion of the reciprocating elements are substantially parallel with the path of motion, and symmetrical.
In addition, it is essential to the invention that the clearance between the gear rack and the gear wheel has been substantially eliminated by means of a support roll with bearings, said roll being arranged to act at the point of contact of the gear, on the back of the gear rack.
According to the present invention, an lightly moving actuator is provided, where delay effects caused by clearances or by wall friction within the cylinder-piston system are substantially eliminated. The actuator can be implemented either as double-acting, whereby pressurized spaces work in opposite directions, or as single-acting, whereby the pressurized spaces at both ends of the gear rack work in the same direction, and return is effected by a spring. In case of a single-acting device, one end of the gear rack requires a sealed through-hole, but as the force moving the gear rack is strong due to the double piston area, the friction of the bushing has no great significance for the mobility.